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Voids in a solder joint can reduce their fatigue life. Voids are caused by the presence of flux in solder paste during reflow soldering, and they are difficult to remove completely. From numerical analysis, we aimed to obtain criteria for evaluating the effect of voids on fatigue life in a solder joint. We investigated crack propagation in the microsolder joints of a semiconductor and developed a new crack propagation model. In our model, the fatigue life of a solder joint is evaluated based on damage that is accumulated during crack propagation, and crack paths are automatically calculated. The crack-propagation behavior of a center-cracked-plate (CCP) specimen calculated using the model agreed well with that obtained from measurement. Using our model, we analyzed the effect of positions and sizes of voids on crack paths and the fatigue life of a ball grid array (BGA) structure. The crack paths and the fatigue life were both found to strongly depend on the positions and sizes of voids. We have achieved a reliable method of evaluating the effects of voids in a solder joint.